1. Field of the Invention
The present invention relates to a plate clamping unit for use in an offset press. More particularly, the present invention pertains to a plate clamping unit which is able to adequately clamp an end of a printing plate mounted on a plate cylinder.
2. Description of the Related Art
FIG. 25 shows a general mechanical composition of an offset press machine. The offset press includes a group of rollers, a plate cylinder 200, a blanket cylinder 210, and an impression cylinder 211. Ink and water are fed from a group of inking rollers 212 and moistening rollers 213, respectively. The ink and the water are mixed and applied to the plate cylinder 200. Then, the mixture of ink and water is applied to a printing plate 204 mounted on the plate cylinder 200. The ink over the printing plate 204 is transferred to a blanket 214 on the blanket cylinder 210. Then, the ink over the blanket 214 is printed on printing sheets Pa on the impression cylinder 211. Thus, the state of the printing plate is printed on the sheets Pa at the second transfer.
Therefore, the setting state of the printing plate 204 around the plate cylinder 200 is a decisive factor of printing quality.
As shown in FIGS. 22 and 23, a groove 201 is formed on the outer peripheral surface of the plate cylinder 200. The groove 201 extends perpendicularly to the drawing sheet. A first and a second plate clamps 202, 203 are mounted in the groove 201 to clamp the top and bottom ends of the printing plate 204, respectively. The plate clamps 202, 203 are forcibly moved in the radial direction with respect to the plate cylinder 200 for the opening motion. The holding portions of the clamps 202, 203 are moved in the peripheral direction of the plate cylinder 200 for the opening motion. The printing plate 204 is made of a thin aluminum plate the both end of which are folded. The folded ends of the plate 204 are adequately placed in holding portions to be securely clamped by the clamps 202, 203. Thus, the printing plate 204 is held on the outer periphery of the plate cylinder 200.
As shown in FIG. 23, the first clamp 202 is fixed to the plate cylinder 200. A screw 207 is provided to adjust the distance between a pair of second plate clamps 205 and 206. A pair of torsion adjusting screws 208 and 209 is disposed in the distal end of the second clamps 205 and 206 so as to be advanced or retreated. The distal end of each torsion adjusting screws 208, 209 is in contact with the inner wall of the groove 201, respectively.
When the printing plate 204 is mounted on the plate cylinder 200, with some torsion as shown in the chain line in FIG. 23, the torsion can be removed by retreating the screw 208 and advancing the screw 209 to move the second clamps 205, 206 for expanding the end of the plate 204.
As shown in FIG. 24, in the offset press, the printing sheet Pa on the impression roller 211 receives printing pressure and a small amount of water. This causes to expand an end of each sheet Pa, as shown by chain line, when the multicolor printing is carried out. This end expansion is called "fanout" by the operators. If further printing is made on the printing sheet Pa, the printed images would shrink as the printing sheet Pa dries and shrinks to the original size. To counter this fanout, the screws 208, 209 are advanced, the screw 207 being manipulated to widen the distance between the clamps 205, 206 and expanding the end of the plate 204.
However, it is time consuming and tiresome to fold the ends of the printing plate 204. Furthermore, for folding a printing plate made of aluminum, a dedicated folding machine is required, resulting in the high cost.
Moreover, the two screws 208, 209 at the both ends of the clamp 203 must be manipulated by turning to adjust the torsion of the printing plate 204. The screws 208 and 209 at the both ends of the clamps 205, 206 and the screw 207 between the clamps 205, 206 must be alternately manipulated for preventing the sheet Pa from the fanout. This troublesome adjusting operation reduces the efficiency of the work.